1. Field of the Invention
The present invention relates to a two-component developer for developing latent electrostatic images to toner images for use in electrophotography in a copying machine, facsimile apparatus or the like, and also for use in electrostatic recording, and electrostatic printing process.
2. Discussion of Background
Various electrophotographic methods are described, for instance, in U.S. Pat. No. 2,297,691, Japanese Patent Publication 42-23910, and Japanese Patent Publication 43-24748. In these methods, a photoconductive layer is irradiated with a light image corresponding to an original image to be copied, and a latent electrostatic image corresponding to the original image is formed on the photoconductive layer. In a normal development, a toner with an opposite polarity to that of the latent electrostatic image is deposited on the electrostatic image to develop the latent electrostatic image to a toner image on the photoconductive layer, and when necessary, the toner image is transferred to an image transfer material such as a sheet of paper, and the transferred toner image is then fixed thereto with the application of heat, pressure, heat and pressure, or a solvent vapor thereto, whereby a copied material can be obtained.
In the step of developing the latent electrostatic image to the toner image, the toner image is formed, utilizing the electrostatic attraction between charged toner particles and the latent electrostatic image. Generally, in the above-mentioned development method of developing the electrostatic image with the toner, a two-component developer comprising toner particles and carrier particles is preferably employed in a copying machine for which high image quality is demanded.
The carrier particles in the two-component developer are constantly stirred while in use for an extended period of time, so that the surface of the carrier particles is apt to be damaged and therefore it is difficult to attain stable triboelectric charging over an extended period of time.
Furthermore, a photoconductor drum, which is disposed so as to face a development sleeve for carrying the two-component developer thereon, is also abraded with the two-component developer, and a cleaning blade is brought into pressure contact with the photoconductor drum, so that the surface of the photoconductor drum is caused to deteriorate with time, and the image quality is also caused to deteriorate.
Japanese Laid-Open Patent Application 62-66268 discloses a carrier comprising carrier particles coated with a coating layer comprising a silicone-oil-containing silicone resin. In this prior art reference, it is stated that by the provision of the coating layer, the triboelectric charging characteristics of the carrier are improved, whereby images with such an image quality that is free of fogging can be obtained.
However, the silicone-oil containing carrier disclosed in the above-mentioned prior art reference, however, has the problems that the fluidity of the carrier is caused to deteriorate due to the presence of an excessive amount of the silicone oil in the surface layer of the carrier particles and therefore the toner cannot be appropriately charged by the carrier, so that the fogging is apt to take place.
Recently, in order to meet the demand for high quality image, an appropriate combination of the carrier and toner particles with a small particle diameter is essential. However, the above-mentioned prior art reference does not disclose anything about the combination of the carrier and a small-particle diameter toner, and conventionally, high quality images cannot be obtained when a small-particle diameter toner is employed.
In the above-mentioned prior art reference, there is disclosed a two-component developer composed of the carrier and toner particles with a particle diameter as small as 6.5 .mu.m. The inventors of the present invention evaluated the developer. The result was that images obtained have such an image quality that has extreme fogging. The inventors of the present invention investigated the cause of the deterioration of the image quality with the extreme fogging and discovered that it is caused because the fluidity of the carrier is poor and the toner cannot be charged with an appropriate amount of triboelectric charges by the carrier. However, it is extremely difficult to provide a sharp charge quantity distribution by using small-particle diameter toners which are currently employed and the above-mentioned carrier because the triboelectric charging area in the small-particle diameter toners is large and the fluidity thereof is poor.